Expressions

Expressions are pieces of sql that return a value when the database interprets them. The Dart API from drift allows you to write most expressions in Dart and then convert them to sql. Expressions are used in all kinds of situations. For instance, where expects an expression that returns a boolean.

In most cases, you're writing an expression that combines other expressions. Any column name is a valid expression, so for most where clauses you'll be writing a expression that wraps a column name in some kind of comparison.

Comparisons

Every expression can be compared to a value by using equals. If you want to compare an expression to another expression, you can use equalsExpr. For numeric and datetime expressions, you can also use a variety of methods like isSmallerThan, isSmallerOrEqual and so on to compare them:

// find all animals with less than 5 legs:
(select(animals)..where((a) => a.amountOfLegs.isSmallerThanValue(5))).get();

// find all animals who's average livespan is shorter than their amount of legs (poor flies)
(select(animals)..where((a) => a.averageLivespan.isSmallerThan(a.amountOfLegs)));

Future<List<Animal>> findAnimalsByLegs(int legCount) {
  return (select(animals)..where((a) => a.legs.equals(legCount))).get();
}

Boolean algebra

You can nest boolean expressions by using the &, | operators and the not method exposed by drift:

// find all animals that aren't mammals and have 4 legs
select(animals)..where((a) => a.isMammal.not() & a.amountOfLegs.equals(4));

// find all animals that are mammals or have 2 legs
select(animals)..where((a) => a.isMammal | a.amountOfLegs.equals(2));

If you have a list of predicates for which one or all need to match, you can use Expression.or and Expression.and, respectively:

Expression.and([
  a.isMammal,
  a.amountOfLegs().equals(4),
])

Arithmetic

For int and double expressions, you can use the +, -, * and / operators. To run calculations between a sql expression and a Dart value, wrap it in a Variable:

Future<List<Product>> canBeBought(int amount, int price) {
  return (select(products)..where((p) {
    final totalPrice = p.price * Variable(amount);
    return totalPrice.isSmallerOrEqualValue(price);
  })).get();
}

String expressions define a + operator as well. Just like you would expect, it performs a concatenation in sql.

For integer values, you can use ~, bitwiseAnd and bitwiseOr to perform bitwise operations:

Expression<int> bitwiseMagic(Expression<int> a, Expression<int> b) {
  // Generates `~(a & b)` in SQL.
  return ~(a.bitwiseAnd(b));
}

Nullability

To check whether an expression evaluates to NULL in sql, you can use the isNull extension:

final withoutCategories = select(todos)..where((row) => row.category.isNull());

The expression returned will resolve to true if the inner expression resolves to null and false otherwise. As you would expect, isNotNull works the other way around.

To use a fallback value when an expression evaluates to null, you can use the coalesce function. It takes a list of expressions and evaluates to the first one that isn't null:

final category = coalesce([todos.category, const Constant(1)]);

This corresponds to the ?? operator in Dart.

Date and Time

For columns and expressions that return a DateTime, you can use the year, month, day, hour, minute and second getters to extract individual fields from that date:

select(users).where((u) => u.birthDate.year.isSmallerThanValue(1950));

The individual fields like year, month and so on are expressions themselves. This means that you can use operators and comparisons on them. To obtain the current date or the current time as an expression, use the currentDate and currentDateAndTime constants provided by drift.

You can also use the + and - operators to add or subtract a duration from a time column:

Future<void> increaseDueDates() async {
  final change = TodoItemsCompanion.custom(
      dueDate: todoItems.dueDate + Duration(days: 1));
  await update(todoItems).write(change);
}

For more complex transformations of a datetime, the modify and modifyAll function is useful. For instance, this increments every dueDate value for todo items to the same time on a Monday:

Future<void> moveDueDateToNextMonday() async {
  final change = TodoItemsCompanion.custom(
      dueDate: todoItems.dueDate
          .modify(DateTimeModifier.weekday(DateTime.monday)));
  await update(todoItems).write(change);
}

IN and NOT IN

You can check whether an expression is in a list of values by using the isIn and isNotIn methods:

select(animals)..where((a) => a.amountOfLegs.isIn([3, 7, 4, 2]);

Again, the isNotIn function works the other way around.

Aggregate functions (like count and sum)

Aggregate functions are available from the Dart api. Unlike regular functions, aggregate functions operate on multiple rows at once. By default, they combine all rows that would be returned by the select statement into a single value. You can also make them run over different groups in the result by using group by.

Comparing

You can use the min and max methods on numeric and datetime expressions. They return the smallest or largest value in the result set, respectively.

Arithmetic

The avg, sum and total methods are available. For instance, you could watch the average length of a todo item with this query:

Stream<double> averageItemLength() {
  final avgLength = todos.content.length.avg();

  final query = selectOnly(todos)
    ..addColumns([avgLength]);

  return query.map((row) => row.read(avgLength)).watchSingle();
}

Note: We're using selectOnly instead of select because we're not interested in any colum that todos provides - we only care about the average length. More details are available here

Counting

Sometimes, it's useful to count how many rows are present in a group. By using the table layout from the example, this query will report how many todo entries are associated to each category:

final amountOfTodos = todos.id.count();

final query = db.select(categories).join([
  innerJoin(
    todos,
    todos.category.equalsExp(categories.id),
    useColumns: false,
  )
]);
query
  ..addColumns([amountOfTodos])
  ..groupBy([categories.id]);

If you don't want to count duplicate values, you can use count(distinct: true). Sometimes, you only need to count values that match a condition. For that, you can use the filter parameter on count. To count all rows (instead of a single value), you can use the top-level countAll() function.

More information on how to write aggregate queries with drift's Dart api is available here

group_concat

The groupConcat function can be used to join multiple values into a single string:

Stream<String> allTodoContent() {
  final allContent = todos.content.groupConcat();
  final query = selectOnly(todos)..addColumns(allContent);

  return query.map((row) => row.read(query)).watchSingle();
}

The separator defaults to a comma without surrounding whitespace, but it can be changed with the separator argument on groupConcat.

Mathematical functions and regexp

When using a NativeDatabase, a basic set of trigonometric functions will be available. It also defines the REGEXP function, which allows you to use a REGEXP b in sql queries. For more information, see the list of functions here.

Subqueries

Drift has basic support for subqueries in expressions.

Scalar subqueries

A scalar subquery is a select statement that returns exactly one row with exactly one column. Since it returns exactly one value, it can be used in another query:

Future<List<Todo>> findTodosInCategory(String description) async {
  final groupId = selectOnly(categories)
    ..addColumns([categories.id])
    ..where(categories.description.equals(description));

  return select(todos)..where((row) => row.category.equalsExp(subqueryExpression(groupId)));
}

Here, groupId is a regular select statement. By default drift would select all columns, so we use selectOnly to only load the id of the category we care about. Then, we can use subqueryExpression to embed that query into an expression that we're using as a filter.

isInQuery

Similar to isIn and isNotIn functions, you can use isInQuery to pass a subquery instead of a direct set of values.

The subquery must return exactly one column, but it is allowed to return more than one row. isInQuery returns true if that value is present in the query.

Exists

The existsQuery and notExistsQuery functions can be used to check if a subquery contains any rows. For instance, we could use this to find empty categories:

Future<List<Category>> emptyCategories() {
  final hasNoTodo = notExistsQuery(select(todoItems)
    ..where((row) => row.category.equalsExp(categories.id)));
  return (select(categories)..where((row) => hasNoTodo)).get();
}

Full subqueries

Drift also supports subqueries that appear in JOINs, which are described in the documentation for joins.

Custom expressions

If you want to inline custom sql into Dart queries, you can use a CustomExpression class. It takes a sql parameter that lets you write custom expressions:

const inactive = CustomExpression<bool, BoolType>("julianday('now') - julianday(last_login) > 60");
select(users)..where((u) => inactive);

Note: It's easy to write invalid queries by using CustomExpressions too much. If you feel like you need to use them because a feature you use is not available in drift, consider creating an issue to let us know. If you just prefer sql, you could also take a look at compiled sql which is typesafe to use.